Allow building src/3rdparty projects instead of using vcpkg

Initially a decision was made to move away from bundling 3rd party source code in Qt 6, regardless of which build system is used, and instead leverage something like Conan or vcpkg.

This proved to be an involved task (required fork of vcpkg, will need additional work in Qt Installer and possibly Coin), and there's lots of other stuff left to do.

Instead, for the time, being we got approval to proceed with bundling / building src/3rdparty projects.

This means we should:

• allow the configure system to choose whether to use system libraries or build and use Qt provided ones
• make sure to export proper CMake packages and qmake pri files
• have appropriate WrapFoo.cmake files that allow choosing system / qt libraries